hi I am user of fluent for 2 years. The present problem is as follows: a closed cylindrical tank in which liquid hydrogen is partially filled(cryogenic propellant tank). Now, since the liquid will be stored at 20k,there will be heat leakage into the tank. so there has to be evaporation from liquid to the ullage space... I am solving this problem in fluent using VOF model and there is no option for evaporation..so presently i am trying using UDF for mass transfer at phase interaction panel. I am struggling to come out with a perfect equation including vapor pressure. So can you help me in this regard. thanks in advance prasanth